Currently utilized technologies for providing protective coatings, such as electromagnetic interference shielding, can be divided into 2 categories: a) metallic shields or casings; and b) thin metal coatings. The metallic shields or casings are formed metal parts that sit over specific electrical components to be shielded. Alternatively, larger metal parts may be used to cover the entire electronic circuitry involved. Extra processing steps are required in the manufacturing of such metal parts, often requiring a complicated geometry for smaller single component shields. The larger metallic shields which cover the entire electronics boards are simpler to manufacture and install,but increase the weight of the completed electronic device. In either case, additional costs are added to the manufacturing process for the cost of the metal, fabrication to the desired shape, and mounting in the electronic device.
An alternative approach, which is currently used instead of separate metallic shielding, is to spray a thin coating of metal particles onto the interior surface of the casing of the electronic device. This material, which can be applied in a similar fashion to paint, contains very fine metallic particles (typically nickel) which form a shielding layer. The metal particles must be extremely fine to allow spraying, and hence, the coating is costly. Also, the metallic film must be of uniform thickness and adhere well to the substrate onto which it is sprayed in order to allow sufficient percolation to adequately shield. The major disadvantage with sprayed metallic coatings is that the usually plastic casing of the electronic device is rendered non-recyclable. The nickel (or other metal) which must be intimately bonded to the casing of the electronic device to assure sufficient EMI shielding prevents recycling of the plastic casing, as the metal will degrade the plastic if recycling is attempted.
It is known in the art to utilize small, high aspect ratio conducting Acylinders such as carbon microfibrils or nanotubes as coatings or in dispersion to fabricate electromagnetic shielding materials. For example, U.S. Pat. No. 5,853,877 to Shibuta discloses treatment of carbon microfibril agglomerates with sulfur-containing strong acids such as sulfuric acid and an oxidizing agent such as nitric acid, to disentangle the nanotubes prior to dispersion in polar solvents, for use in forming transparent electrically conductive films. This disentangling step by treatment with sulfur-containing strong acids and oxidizing agents is specifically required in the Shibuta process to allow formation of an electrically conductive film of sufficient transparency.
Similarly, U.S. Pat. No. 5,908,585 to Shibuta discloses transparent electrically conductive films comprising 0.01%-1 wt. % of hollow carbon microfibers and 1%-40 wt. % of an electrically conductive metal oxide powder. The inclusion of relatively high percentages of metal oxide is required in the method of the '585 patent to allow suitable levels of electrical conductivity without impairing the transparency of the electrically conductive films created thereby by high concentrations of carbon microfibers.
The electrically conductive films described in the '877 and '585 patents to Shibuta are generally effective for their intended purposes. However, the films suffer from certain disadvantages. In the '877 patent, sulfur-containing strong acids and oxidizing agents are required to disentangle the carbon microfibers to form a suitable electrically conductive film. Accordingly, separate disentangling and dispersion steps are required prior to coating the desired surface with the electrically conductive film of the '877 patent. Further, the harsh oxidative treatment required to disentangle the carbon microfibrils alters the surface characteristics and chemistry of the microfibrils. Specifically, such treatment of carbon microfibrils is known to shorten them, reducing the aspect ratio, and thereby requiring increased amounts of microfibrils in a composition to reach a particular percolation threshold (Liu J., Rinzler A. G., Dai H., Hafner J. H., Bradley R. K., Boul P. J., Lu A., Iverson T., Shelimov K., Huffman C. B., Rodriguez-Macias F., Shon Y.-S., Lee T. R., Colbert D. T., Smalley R. E. (1998) Fullerene pipes. Science 280: 1253-1255; incorporated herein by reference). This use of increased quantities of relatively expensive microfibrils to achieve a predetermined percolation threshold substantially increases the production costs of this approach.
Inclusion of metal oxide powders in the compositions of the '585 patent is necessary to impart the desired electrical conductivity to the final product. Addition of metal oxide powder such as antimony-doped tin oxide, however, may render any plastic coated with the composition unsuitable for recycling, and further adds significant cost to the process. The metal oxide powder also weakens the resulting thin film, as the oxide acts as an inert filler within the film. This type of filler reduces the mechanical properties of the film compared to, for example, inclusion of carbon nanotubes alone (P. K. Mallick, 1993. “Fiber Reinforced Composites.” (2d edition), Marcel Dekker, New York, N.Y.; incorporated herein by reference).
Accordingly, there is a need in the art for carbon nanotube-based compositions suitable for use as surface coatings for desired target surfaces or substrates which do not require harsh chemical treatments to allow disentangling and dispersion of the nanotubes. There is further a need in the art for such compositions which do not require the addition of weakening metal oxide fillers to achieve suitable levels of electrical conductivity, and which do not inhibit recycling of the plastic components of electronic devices to which the compositions are applied.